Study of the field hydraulic conductivity of thin open-graded friction courses

Permeability of asphalt pavements is a property of interest due to its impact on durability and, potentially, safety. In the case of open-graded friction courses (OGFC), the focus with permeability is on the improved drainage capacity that reduces splash and spray, and hydroplaning potential. Multiple research efforts have developed permeameters to evaluate field permeability. However, most of these efforts consider unidirectional linear Darcy flow, which is not sufficient to describe flow through OGFC layers during testing conditions. The objective of this investigation was to examine the use of an approximate analytical solution based on 2D flow proposed by researchers at UT-Austin to study the field permeability of thin OGFC layers. Additional values for a shape factor were proposed to test relatively large areas of thin OGFC layers. Furthermore, it was found that the use of vacuum grease to prevent water flow between the base of the permeameter and the pavement surface clogged the OGFC surface voids, which notably reduced hydraulic conductivity. To overcome this issue, this study evaluated the use of a closed-cell high-density foam pad as a sealant method. Lastly, the study showed the need for a minimum volume of water to provide a smooth drop in water head during testing. This is necessary to allow for accurate measurement of the three time readings required to describe the nonlinear relationship between elevation head and flow rate during a falling-head permeability test. The lessons learned from this study were summarized in a set of guidelines for the successful development of a new permeameter. These guidelines can be used by other highway agencies interested in evaluating the field hydraulic conductivity of thin OGFC layers.